eyewitness on the scene. Paul ferguson, in london, today. Paul, I understand you're in a building across the way. What exactly did you see? Well, a few minutes before 8:00 our local time, I was looking out the window, and thought, what's going on there? There's a flash in the sky. They're building a very tall block of flats, which is apartments, by the river there. And a helicopter had clipped the crane on the very top of the building and then plummeted immediately to the ground, exploding. We know that the pilots were warned about the crane in that area. Could you detect any attempt by the helicopter to evade the crane? When I looked up, no. It was -- its propeller wings or its foot plate had just snatched on to the side of the crane. It was a typical london wintry morning. It was very, very foggy. The mist was covering the building. And I didn't see the usual red safety lights glowing in the dark that would advise the helicopter's proximity to the building. And what happened exactly after contact? After contact, suddenly, it just plummeted to the ground and exploded. And huge plumes of smoke came up. Fortunately, or rather unfortunately for the pilot, it was a straight drop down to a main arterial road to the city. But it did avoid a lot of the surrounding buildings, a lot commercial premises and residential premises in that area. Paul ferguson, thank you for your time this morning.

This transcript has been automatically generated and may not be 100% accurate.

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